Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) $ in Millions		Mar. 31, 2025 USD ($)		Dec. 31, 2024 USD ($)
Contingent consideration [Member] | Fair Value Inputs, Discount Rate [Member]
Liabilities:
Fair value inputs, weighted-average discount rate		0.1341		0.1343
Contingent consideration [Member] | Fair Value Inputs, Discount Rate [Member] | Minimum [Member]
Liabilities:
Fair value inputs, weighted-average discount rate		0.11
Contingent consideration [Member] | Fair Value Inputs, Discount Rate [Member] | Maximum [Member]
Liabilities:
Fair value inputs, weighted-average discount rate		0.138
Recurring [Member]
Assets:
Mutual funds / exchange traded funds	[1]	$ 109		$ 108
Derivative financial instruments	[2]	3		1
Liabilities:
Contingent consideration	[4]	40	[3]	39
Derivative financial instruments	[2]	2		6
Recurring [Member] | Commingled Funds [Member]
Assets:
Deferred compensation plans	[1],[5]	18		18
Recurring [Member] | Hedge Funds [Member]
Assets:
Deferred compensation plans	[1]	18		17	[6]
Recurring [Member] | Certificates of Deposit/Term Deposit [Member]
Assets:
Short-term investment (held to maturity)	[7]	30
Recurring [Member] | Fiduciary Assets [Member]
Assets:
Mutual funds / exchange traded funds		336		337
Recurring [Member] | Level 1 [Member]
Assets:
Mutual funds / exchange traded funds	[1]	109		108
Derivative financial instruments	[2]	0		0
Liabilities:
Contingent consideration	[4]	0	[3]	0
Derivative financial instruments	[2]	0		0
Recurring [Member] | Level 1 [Member] | Commingled Funds [Member]
Assets:
Deferred compensation plans	[1],[5]	0		0
Recurring [Member] | Level 1 [Member] | Hedge Funds [Member]
Assets:
Deferred compensation plans	[1]	0		0	[6]
Recurring [Member] | Level 1 [Member] | Certificates of Deposit/Term Deposit [Member]
Assets:
Short-term investment (held to maturity)	[7]	30
Recurring [Member] | Level 1 [Member] | Fiduciary Assets [Member]
Assets:
Mutual funds / exchange traded funds		336		337
Recurring [Member] | Level 2 [Member]
Assets:
Mutual funds / exchange traded funds	[1]	0		0
Derivative financial instruments	[2]	3		1
Liabilities:
Contingent consideration	[4]	0	[3]	0
Derivative financial instruments	[2]	2		6
Recurring [Member] | Level 2 [Member] | Commingled Funds [Member]
Assets:
Deferred compensation plans	[1],[5]	0		0
Recurring [Member] | Level 2 [Member] | Hedge Funds [Member]
Assets:
Deferred compensation plans	[1]	0		0	[6]
Recurring [Member] | Level 2 [Member] | Certificates of Deposit/Term Deposit [Member]
Assets:
Short-term investment (held to maturity)	[7]	0
Recurring [Member] | Level 2 [Member] | Fiduciary Assets [Member]
Assets:
Mutual funds / exchange traded funds		0		0
Recurring [Member] | Level 3 [Member]
Assets:
Mutual funds / exchange traded funds	[1]	0		0
Derivative financial instruments	[2]	0		0
Liabilities:
Contingent consideration	[4]	40	[3]	39
Derivative financial instruments	[2]	0		0
Recurring [Member] | Level 3 [Member] | Commingled Funds [Member]
Assets:
Deferred compensation plans	[1],[5]	0		0
Recurring [Member] | Level 3 [Member] | Hedge Funds [Member]
Assets:
Deferred compensation plans	[1]	0		0	[6]
Recurring [Member] | Level 3 [Member] | Certificates of Deposit/Term Deposit [Member]
Assets:
Short-term investment (held to maturity)	[7]	0
Recurring [Member] | Level 3 [Member] | Fiduciary Assets [Member]
Assets:
Mutual funds / exchange traded funds		$ 0		$ 0

[1] With the exception of the funds included in fiduciary assets, the majority of these balances are held as part of deferred compensation plans with related liabilities in other current liabilities and other non-current liabilities on the condensed consolidated balance sheets.

[2] See Note 9 — Derivative Financial Instruments for further information on our derivative investments.

[3] Consideration due to be paid across multiple years until 2029.

[4] Probability weightings are based on our knowledge of the past and planned performance of the acquired entity to which the contingent consideration applies. The fair value weighted-average discount rates used in our material contingent consideration calculations were 13.41% and 13.43% at March 31, 2025 and December 31, 2024, respectively. The range of these discount rates was 11.00% - 13.80% at March 31, 2025. Using different probability weightings and discount rates could result in an increase or decrease of the contingent consideration payable.

[5] Consists of the Towers Watson Global Equity Focus Fund, for which redemptions can occur on any business day, and require a minimum of one business day’s notice.

[6] Consists of the Towers Watson Alternative Credit Fund, for which the redemption period is generally quarterly, however requires a 50-day notice.

[7] Consists of investments with maturity dates of up to 90 days.